The invention is based on a fuel injection system for internal combustion engines as defined hereinafter. In one such fuel injection system, known from German Offenlegungsschrift 41 15 103, which serves to supply fuel to an internal combustion engine, a high-pressure fuel pump embodied as a piston pump fills a pressure reservoir chamber with fuel via a high-pressure line. From this pressure reservoir chamber, fuel injection lines lead to the individual injection valves, which communicate with one another (common rail) via the pressure reservoir chamber; the pressure reservoir is kept at a predetermined pressure by a pressure control device, so that at the injection valves the injection pressure can be fixed independently of rpm over the entire operating performance graph of the engine to be supplied.
The injection valves protruding into the combustion chamber of this engine have a pistonlike valve member, axially guided in a guide bore in the valve housing, whose one face end has a conical sealing face that cooperates with a valve seat and whose other face end defines a pressure chamber in the guide bore in the valve housing; the pressure chamber can be made to communicate with an injection pressure chamber surrounding the valve member via a connecting line containing a throttle and with a return line via a relief line containing an electric control valve. The injection pressure chamber communicates with the pressure reservoir chamber via a high-pressure fuel line and always has the same pressure level as the reservoir chamber. The valve member has a conical cross-sectional constriction in the direction of the valve sealing face in the region of the injection pressure chamber, and when the control valve is closed and there is a pressure equilibrium between the face-end pressure chamber and the injection pressure chamber, is held by its valve sealing face in contact with the valve seat by a valve spring, so that in this region the injection ports are closed.
The onset of the opening stroke of the valve member is initiated by the electrical opening of the control valve in the relief line from the face-end pressure chamber; as a result, the high pressure in the face-end pressure chamber decreases rapidly, so that a pressure drop arises between it and the injection pressure chamber; this pressure drop causes an opening stroke motion of the valve member counter to the force of the valve spring, and the throttle in the connecting line prevents a rapid return flow of fuel into the face-end pressure chamber. Analogously, the closure of the injection valve is controlled via the closure of the control valve; a high fuel pressure builds up again in the face-end pressure chamber, reinforcing the closing force of the valve spring and presses the valve member back onto its valve seat. When the control valve is without current or in other words closed, the valve member is hydraulically kept in its contact with the valve seat by the pressure in the face-end pressure chamber.
To avoid influence on the injection pressure in the filling and high-pressure chamber of the injection valve from the opening of the face-end pressure chamber, a further fuel injection system of this generic type is known from the special issue of ATZ/MTZ called Motor und Umwelt 92 [Motor and Environment 92], in which the face-end pressure chamber can be made to communicate via a three-way valve with the common pressure chamber (common rail) or a relief line. There, filling of the face-end control pressure chamber takes place unthrottled via a check valve, and the relief for opening the injection valve is throttled. The injection pressure in the injection valve is not affected in any way by the opening of the control pressure chamber, since the communication between the high-pressure line system and the face-end control pressure chamber is closed at that moment.
Both of the known fuel injection systems, however, have the disadvantage that the very high fuel injection pressure must be generated in a high-pressure fuel pump that communicates with the common pressure chamber via high-pressure lines and also communicates with the individual injection valves. The result is that in terms of strain on components and design, very stringent demands must be made of the high-pressure pump and the line system, and this dictates complicated, expensive manufacture. Furthermore, because of the relatively long transmission path from the high-pressure pump to the injection location, pressure losses arise, and so the known fuel injection systems are unable to attain very high injection pressures of up to 2000 bar. A further disadvantage of the known fuel injection systems arises from the use of the fuel, which is at high injection pressure, as a control pressure means, so that above all during closure at the end of injection, the control valves must bring to bear relatively high adjusting forces, which in turn dictates large dimensions and hence longer switching times.